1. Field of the Invention
The present invention relates to a photomask blank, and more particularly to a photomask blank for manufacturing a photomask used in microfabrication of a semiconductor integrated circuit, a CCD (charge-coupled devices), a color filter for a liquid crystal display (LCD), a magnetic head and the like.
2. Description of the Related Art
In the field of semiconductor processing technology in recent years, there has been a demand for increasing miniaturization of circuit patterns to accompany increasing integration of large scale integrated circuits. To achieve such miniaturization, demands with respect to technology for thinning lines of wiring patterns that constitute such circuits as well as technology for miniaturizing contact hole patterns used for wiring between layers making up a cell are also becoming more severe. Consequently, in the manufacture of photolithography photomasks for forming these wiring patterns and contact hole patterns also, there is a demand for technology capable of writing finer circuit patterns with greater accuracy.
It is known that if a change in the shape of a photomask occurs when a pattern that has been drawn on the photomask is projected onto a resist film, the accuracy of the projection position of the pattern will decrease and result in a defective product. It has been clarified that it is necessary to control the substrate shape of the photomask to solve this problem (Patent Literature 1: Japanese Patent Laid-Open No. 2003-50458). Patent Literature 1 reports that by using a substrate having a specific surface shape as the substrate for manufacturing a photomask, a change in the surface shape can be suppressed when the photomask is fixed by suction on a mask stage of an exposure apparatus.
Conventionally, importance has been placed on the flatness of a transparent substrate for a photomask and a photomask blank. When an optical film such as a light-shielding film or a phase shift film is deposited on a transparent substrate for a photomask, the stress in the optical film is controlled so that the substrate shape does not change. Many reports exists regarding technology for suppressing the occurrence of “warpage”, that is, a change in the shape of the substrate surface (for example, Patent Literature 2: Japanese Patent Laid-Open No. 2004-199035).
In addition, to correspond to further thinning of patterns, liquid immersion exposure or double patterning as well as technologies that combine these are in practical use. Here, the term “liquid immersion exposure” refers to a method that increases the accuracy of patterning by filling a space between the lens of a semiconductor exposure apparatus and a substrate that is the object of exposure (for example, a silicon wafer) with a liquid (for example, pure water with a refractive index of 1.44) that has a higher refractive index than air (refractive index of 1.00). The term “double patterning” refers to a method in which exposure is performed after dividing one circuit pattern into two low-density circuit patterns, in which the aforementioned two patterns are transferred in sequence onto a substrate. That is, after one of the circuit patterns is transferred, the other circuit pattern is transferred between the lines of the first circuit pattern, to thereby achieve a higher resolution.
For example, in a photomask for double patterning that corresponds to hp 32, approximately 3 to 4 nm is required as the superposition accuracy when exposing a resist that has been coated on a substrate. In addition to this requirement for a high degree of superposition accuracy, since the exposure light is short-wavelength light with a wavelength of 193 nm and the depth of focus (DOF) is therefore shallow, the photomask is required to have a high degree of flatness in order to achieve highly accurate patterning.
A phase shift method is available as one method that enhances the resolution without making the depth of focus shallow. According to the phase shift method, a photomask (phase shift mask) is used on which a pattern is formed so that there is a phase difference of approximately 180° between light that has passed through a phase shift section and light that has passed through a transparent section. Light that has passed through the phase shift section and light that has passed through the transparent section interfere with each other to form transmitted light with opposite phases. The phase shift method utilizes phase information of the transmitted light to improve the resolution performance. Note that, the types of phase shift masks include a Levenson-type mask and a halftone-type mask. Halftone phase shift masks have a comparatively simple structure and are widely use.
Masks that have a phase shift film made of molybdenum silicide oxide (MoSiO), molybdenum silicide oxynitride (MoSiON), or molybdenum silicide nitride (MoSiN) are used as halftone phase shift masks (Patent Literature 3: Japanese Patent Laid-Open No. 07-140635).
Such phase shift masks are formed by forming a pattern by electron beam lithography or photolithography on a principal surface of a phase shift mask blank. More specifically, a resist is coated on a phase shift film of the phase shift mask blank, a desired portion of the resist is exposed by electron beams or ultraviolet light, thereafter the resist is developed to obtain a resist pattern, and thus a phase shift film at the desired portion is exposed. Subsequently, the exposed portion of the phase shift film is removed by etching utilizing the resist pattern as a mask to expose the substrate face, and thereafter the resist pattern is peeled off to obtain the phase shift mask.
Generally, in a photomask blank, a functional transparent film such as a phase shift film is formed by sputtering. However, even when using a transparent substrate that exhibits a high degree of flatness when mounted on an exposure apparatus for use for manufacturing a photomask blank, a stress arises in the film during the film deposition process, and the stress causes strain of the substrate and induces warpage of the photomask blank. Further, when stress that has accumulated within a phase shift film or the like is released during the photomask manufacturing process, the warpage state of the substrate changes from the initial state of the selected transparent substrate due to release of the stress.
When this kind of change in the warpage amount of a substrate occurs during the photomask manufacturing process, it results in the loss of the flatness of the photomask at the time of exposure, and the effective DOF with respect to the pattern exposure decreases and becomes a factor that induces a resolution failure.
The above problem does not arise if film deposition is carried out under conditions such that stress within a functional transparent film such as a phase shift film becomes the same level as in other functional films. However, it is extremely difficult, and practically impossible, to find film deposition conditions which satisfy various characteristics required for a functional transparent film and which, at the same time, are also conditions for depositing a low stress film. Therefore, after depositing a film under conditions which satisfy various characteristics required for a functional transparent film, it is necessary to provide a further step for lowering the stress of the film.
The application of energy from outside is known as means for reducing such kind of in-film stress. A hot plate, a heater, a halogen lamp, an infrared lamp and the like can be used as specific energy applying means. When these means are used, if there is a large amount of stress accumulated in the functional transparent film it is necessary to apply an extremely large amount of heat energy to adequately release the stress. However, if the amount of heat energy that is applied to too great, the characteristics of the functional transparent film change and the functional transparent film does not perform its original function.
This disadvantage can be avoided by applying heat energy instantly by means of laser annealing. However, because laser light is light of a single wavelength, there is the problem that the result thereof depends on the light absorption rate that the functional transparent film has with respect to the laser light wavelength. (Patent Literature 4: Japanese Patent Laid-Open No. 2002-229183).
To solve this problem, a method is known that irradiates light from a flash lamp at a predetermined energy density to decrease the stress of a functional transparent film (Patent Literature 2: Japanese Patent Laid-Open No. 2004-199035). According to this method, by controlling an irradiation energy amount of light from a flash lamp to an appropriate amount, it is also possible to make the warpage amount of a halftone phase shift film that is a functional transparent film equal to or less than ±0.05 μm (Patent Literature 5: Japanese Patent Laid-Open No. 2005-331554). Further, after forming this kind of halftone phase shift film with a small warpage amount, a light-shielding film and an antireflection film that are other functional films are deposited to form a laminated body that is employed as a photomask blank.